1. Field of the Invention
The present invention relates to a circuit for monitoring an optical signal level, and more particularly, to a circuit for monitoring an optical signal level which detects a reduction in a received optical signal level and outputs an alarm. In addition, the present invention relates to a method for monitoring the optical signal level and an optical receiver.
2. Description of the Related Prior Art
Conventional circuits for monitoring an optical signal level, which are installed in an optical receiver, monitor an amplitude of a clock extracted from a received signal and output an alarm signal when the value of the amplitude is lowered below a reference level. Such circuits, however, can not exactly detect any optical signal level due to a noise which is generated by an APD (avalanche photodiode), a photoelectric transducer element, when the optical signal level is lowered.
Japanese Patent Application Laid-Open No. 5-37473 discloses another circuit for monitoring the optical signal level. In this circuit, a timing extract circuit extracts a clock signal from an input data signal and sends the clock signal to a first detector, which, in turn, outputs a direct current signal corresponding to the level of the clock signal to a subtracter. On the other hand, a band pass filter outputs, for example, a low frequency band of the data signal to a second detector. The output of the second detector is output to the above described subtracter. The subtracter subtracts the output of the second detector from the output of the first detect or and outputs the result of the subtraction to a comparator. In the case where the input optical level is significantly low, a relatively high noise with respect to the input signal is output from an APD. However, since the levels of the noises output from the first and second detectors respectively are substantially equal, the output signal from the subtracter does not include any noise level, and is stably detected by a comparator. Accordingly, the reduction in the input signal level or break in the input signal can be detected However, this circuit for monitoring the optical signal level comprises t he band pass filter for obtaining a compensating output for preventing malfunction due to the noise. Therefore, this circuit is complex, and since the entire circuit is configured by analog circuits, it is difficult to integrate the circuit.
Therefore, an object of the present invention is to provide a circuit for monitoring optical signal level, which is configured by digital circuits, is readily integrated, and operates with stability. And another object of the present invention is to provide a method for monitoring an optical signal level by using the circuit and an optical receiver using this circuit.
A circuit for monitoring an optical signal level according to the present invention comprises a clock extraction circuit, a noise detection circuit, and an alarm circuit. The clock extraction circuit extracts a clock from an input signal, and the noise detection circuit detects surge noises included in the input signal. And the alarm circuit counts noise pulses output from the noise detection circuit, and outputs an alarm signal based on the number of the counted pulses. The above described noise detection circuit can comprise a frequency multiplier, which multiplies the frequency of the clock and outputs the multiplied clock, and a noise identification circuit, which utilizes the multiplied clock to identify surge noises and outputs a noise detection signal including noise pulses. The alarm circuit can comprise a pulse generator and alarm decision circuit. The pulse generator generates a window pulse having a predetermined period, and the alarm decision circuit counts the noise pulses detected within the period of the window pulse, and outputs an alarm pulse when the number of the counted noise pulses reaches a preset value. In addition, the circuit for monitoring the optical signal level can comprise an alarm protection circuit. The alarm protection circuit expands the period of the window pulse, counts the alarm pulses within the expanded period, and outputs the alarm signal when the number of the counted pulses reaches a preset value. A method for monitoring the optical signal level according to the present invention comprises the following steps: that is, detecting the surge noises included in the input signal as pulses, counting the number of the pulses within a predetermined period, and outputting the alarm signal when the number of the counted pulses reaches a preset value. The surge noise is a noise having a level which is relatively increased as the optical signal level is lowered. An optical receiver provided with the circuit for monitoring the optical signal level according to the present invention comprises a light receiving element for receiving the input signal light and outputting an electric signal, a control circuit for controlling the light receiving element based on the output of the light receiving element, a clock extraction circuit for extracting the clock from the output signal from the light receiving element, an identification circuit for identifying the data signal based on the clock, a noise detection circuit for detecting the surge noises included in the input signal, and an alarm circuit for counting the noise pulses output from the noise detection circuit and outputting the alarm signal when the number of the counted pulses reaches a preset value.